The invention relates to oil and natural gas exploration and in particular to methods of locating potential low water cut oil and gas reservoirs in sand formations having significant water saturations.
Low resistivity pay sands are subsurface sand or sandstone formations which indicate low formation resistivity when logged with conventional equipment, implying the presence of a significant amount of water, but produce water-free or relatively low water cut oil or natural gas (i.e. "pay") when tapped. Such sands are located in the Gulf of Mexico and other areas. See for example, E. A. Vajnar, et al "Surprising Productivity from Low Resistivity Sands", 18th Annual Logging Symposium Transactions, Society of Professional Well Logging Analysts, June 5-8, 1977, Houston Tex.; and J. C. Gauntt et al "The Use of Core Analysis Data to Explain the Abnormally Low Resistivities of Some Hydrocarbon-Productive Simpson Series Sand in Central Oklahoma", 4th Annual Logging Symposium Transactions, Society of Professional Well Logging Analysists, May 23-24, 1963, Oklahoma City, Okla. discussing this phenomena. Hereinafter "sand" will be used generically to refer to both consolidated and unconsolidated subsurface geological formations of that material. Also "log" will refer to the signals generated by an appropriate logging tool while traversing the formation, which signals represent measurements of one or more of its varying characteristics as a function of depth, as well as to any temporary record like a CRT display or permanent record like a strip chart, magnetic tape or other computer file of such characteristics or of other varying characteristics derived from the measurements. The log may have values for a continuous range of depths or for a series of discrete depths.
Several methods have been proposed to identify low resistivity pay sands. Basically all involve determining the bound water saturation of the formation from laboratory measurements of samples of the formation material and comparing that to the fractional water saturation determined from the logs. The latter is the total water saturation, bound and free, of the formation expressed as a percent of its pore space. If the former is about equal to or exceeds the latter, it is likely that the water in the formation is bound whereas if the latter exceeds the former, free water exists which will be produced if the formation is tapped at that location. Alternatively, the bound water saturation may be used to determine a so-called "Productive Resistivity" corresponding to that of a low water cut hydrocarbon reservoir which is then compared directly with the formation resistivity log data. This method is described in the aforesaid Gauntt et al article. Gathering and analyzing formation samples is both time consuming and expensive.
The number of samples needed to determine irreducible water saturation of a formation is reduced in some methods by empirically relating the bound or irreducible water saturation measured from a small group of representative formation samples to a characteristic of the formation which can be measured with or determined from conventional logging tools. These methods are based upon the widely held view that bound water saturations are related in some manner to such characteristics of the formation as its porosity, permeability and/or specific surface area (i.e. surface area per quantity of the formation material). Some of these relationships are summarized by A. Timur in an article entitled "An Investigation of Permeability, Porosity, & Residual Water Saturation Relationships for Sandstone Reservoirs", THE LOG ANALYSTS, pp. 8-17, July-August, 1968. One such method, for example, is to correlate irreducible water saturations, surface areas or both measured from rock samples to a formation shaliness factor determined from conventional Spontaneous Potential, gamma ray or density and velocity logs, as is described by Murphy and Owens in "A New Approach for Low-Resistivity Sand Log Analysis", JOUR. OF PETROLEUM TECH., pp. 1302-1306, November 1972.
Some methods dispense with the gathering of formation samples altogether. One is to correlate log-determined fractional water saturations and porosities, as is described at page 104 of a booklet entitled LOG INTERPRETATION Volume I-Principles (1972 ed.), published by Schlumberger Ltd., New York, N.Y. Methods used to determine fractional water saturation and porosity are described elsewhere in the volume which with its contained references is incorporated by reference herein.
Another suggested method which does not require the obtaining of core samples is to measure the free fluid in the formation pore space using a nuclear magnetic resonance tool, such as the nuclear magnetic log manufactured by the aforesaid Schlumberger Ltd. It is assumed in this last method that the remainder of the formation porosity is occupied by bound (i.e. irreducible) fluid, essentially or entirely water.
Each of these methods is multiple stepped in that it requires a resistivity or other log be taken to determine the formation water saturation and one or more additional logs be taken and/or core samples obtained and analyzed before an attempt can be made to identify potential low resistivity pay sands. These multiple step approaches are expensive and time consuming. Furthermore, the last method requires a special logging tool, the nuclear magnetic resonance log, which is not yet commonly used or widely available and may additionally require special treatment of the bore hole mud, adding to exploration expense and effectively restricting its use to those bore holes which were properly preconditioned. Apart from these drawbacks of the presently used methods, it is desirable to have several quick and inexpensive methods to identify potential low resistivity pay sands which may be compared with one another to increase the confidence in identifying such formation zones.